Modulating the nonlinear optical properties of TIDC-Cl by varying the terminal units†

Due to the widespread application of nonlinear optical materials, the design and synthesis of such materials with excellent performance has always been a research hotspot. In this paper, based on the reported TIDC-Cl, five new molecules were designed by introducing donor or acceptor units and their combination. By utilizing density functional theory and time-dependent density functional theory, the linear and nonlinear optical properties of molecules TIDC-Cl and 1–5 were investigated in detail. It was found that except for molecule 1, they are all narrowband compounds (1.41–2.22 eV), and their absorption spectra underwent a red shift, especially for molecules 2 and 4, which have reached the infrared region at long wavelengths. Molecule TIDC-Cl is a candidate for bipolar transport materials and molecules 2–5 are expected to serve as electron transport materials. Given that the studied molecules all have a large first hyperpolarizability, especially molecule 5, which is 807 times larger than that of the organic urea molecule, they are expected to serve as high-performance nonlinear optical materials. In addition, the origin of nonlinearity was studied through DR analysis, hyperpolarizability density analysis and the unit sphere representation method. The discussion of frequency-dependent (hyper)polarizability indicates that the frequency dispersion of the first hyperpolarizability appears in the studied molecules. It is hoped that this study will better promote the understanding of the nonlinear optical response of push–pull materials and pave the way for the development of high-tech nonlinear optical materials.